After hydrocarbons have been removed from the ground, the fluid stream (such as crude oil or natural gas) is transported from place to place via pipelines. It is desirable to know with accuracy the amount of fluid flowing in the stream, and particular accuracy is demanded when the fluid is changing hands, or “custody transfer.” Custody transfer can occur at a fluid fiscal transfer measurement station or skid, which may include key transfer components such as a measurement device or flowmeter, a proving device, associated pipes and valves, and electrical controls. Measurement of the fluid stream flowing through the overall delivery pipeline system starts with the flowmeter, which may include, for example, a turbine meter, a positive displacement meter, an ultrasonic meter, a coriolis meter, or a vortex meter.
Flow characteristics of the fluid stream can change during product delivery, thereby affecting accurate measurement of the product being delivered. Typically, changes of pressure, temperature and flow rate are acknowledged by operator intervention. These changes are represented as changes in the flow characteristics, and are normally verified by the operator via the effects of the changes and their effect on the measurement device. Flow meter performance can be affected by characteristics of the fluid and/or by the arrangement of piping components. Normally, verification of flow meter performance is conducted by proving the flowmeter with a proving device, or prover. A calibrated prover, adjacent the measurement device on the skid and in fluid communication with the measurement device, samples volumes of the fluid and the sampled volumes are compared to the throughput volumes of the measurement device. If there are statistically important differences between the compared volumes, the throughput volume of the measurement device is adjusted to reflect the actual flowing volume as identified by the prover.
The prover has a precisely known volume which is calibrated to known and accepted standards of accuracy, such as those prescribed by the American Petroleum Institute (API) or the International Standards Organization (ISO). The precisely known volume of the prover can be defined as the volume of product between two detector switches that is displaced by the passage of a displacer, such as an elastomeric sphere or a piston. The known volume that is displaced in the prover is compared to the throughput volume of the flowmeter. If the comparison yields a volumetric differential of zero or an acceptable variation therefrom, the flowmeter is then considered to be accurate within the limits of allowed tolerances. If the volumetric differential exceeds the limits allowed, then evidence is provided indicating that the flowmeter may not be accurate. Thereafter, the flowmeter throughput volume can be adjusted to reflect the actual flowing volume as identified by the prover. The adjustment may be made with a flowmeter correction factor. In order to obtain precise calibration, the displacer position in the prover must be accurately determined with dynamic flow of the fluid through the prover. The prover is the in-field reference standard against which the meter is calibrated.